Equipment protection—providing reliable backup service in the case of failure—is an essential part of most high-speed telecommunication systems. Telecommunication equipment typically includes some redundant components, which automatically take over for faulty components when failure occurs. Common protection schemes include 1:1 and 1:N systems. In a 1:1 system, each working communication interface has a dedicated backup interface, also referred to as a protection interface, which remains on standby as long as the working interface is functioning properly. In a 1:N system, a single backup interface serves N working interfaces. The choice of N depends on a tradeoff between cost and reliability demands.
Backplane-based configurations are commonly used in communication and computing equipment. In network access systems, for example, a backplane may be used to connect a main module, having a trunk link to a core network, to a number of subsidiary modules having ports such as DS-3 interfaces, which serve network users (The DS-3 level of the plesiochronous digital hierarchy [PDH] is used in circuit-switched communication networks to carry medium-speed traffic at 44.736 Mbps.) The main and subsidiary modules comprise interface cards, also referred to as line cards, with plug into suitable receptacles, typically edge connectors, on the backplane. Printed circuit traces on the backplane connect the subsidiary modules to the ports of the main module, as well as passing different types of signals (such as data and clock signals) between the line cards located in the same chassis.
Equipment protection in such backplane-based systems typically requires that redundant interface cards be installed in the chassis (also referred to as a shelf). Data signals are conveyed between the redundant cards and the working interface cards using either a box external to the chassis or a protection bus inside the chassis. To avoid awkward and potentially unreliable cabling, the protection bus is best implemented using traces on the backplane. The choice of whether to use 1:1 or 1:N protection is usually made by the network operator based on the cost and reliability constraints of the particular application environment. These constraints may change over time, or when existing equipment is redeployed in a new location or application.
In response to the need for flexible protection configuration, some systems offer the network operator the possibility of configuring the equipment in either a 1:1 or 1:N protection topology. Typically, separate traces are provided on the backplane for the 1:1 and 1:N protection buses, meaning that additional printed circuit traces must be introduced on a board that is already crowded with conductors carrying high-speed signals. Each edge connector on the backplane must also have additional pins to accommodate the different protection buses, and a relay or other switch must be added for selecting the protection mode to be used. It can thus be appreciated that this implementation approach has a number of disadvantages in terms of equipment cost and complexity.